«Pipeline Associated Watercourse Crossings 3rd Edition October 2005 The Canadian Association of Petroleum Producers (CAPP) is the voice of the ...»
Pump discharge locations vary depending upon water quality and the standards for water quality. Bypass water, although usually pumped directly back into the watercourse, was discharged on to the ice downstream of the crossing in one situation. Silty trench water was usually pumped on to shore, either into surrounding vegetation or with sumps, settling ponds or silt fence lined areas. In certain situations, water was discharged into silt bags.
Apart from environmental protection measures relating to the pump discharge areas and bank reclamation, special measures included a full contingency plan in case the crossing was not successful, fish salvage from the isolated areas and secondary upstream dams to trap seepage which in turn was pumped out. Silt curtain or filter fabric/hay bales were installed downstream of the flume with limited success at some crossings.
The total suspended sediment targets were not exceeded during construction of one crossing where monitoring was conducted and results are available.
In general, dam and pump crossings appear to be successful. Those crossings where difficulties were encountered, were the result of poor planning. In particular it is important to: construct high quality impermeable dams; calculate streamflows and have on hand enough pumps for at least 150% of the anticipated flow; have spare generators, fuel and pumps onsite; and finally, a contingency plan in case unforeseen problems arise. One environmental inspector that had
There were several examples of the flumes being installed prior to the crossing construction to allow vehicle access or to avoid instream timing restrictions.
Apart from the environmental protection measures relating to pump discharge, fish salvage between the dams and bank reclamation, no special measures were usually employed. Silt curtain or filter fabric/hay bale dams were installed downstream of the flume at some crossings.
Water quality and sedimentation monitoring was conducted at several crossings where the flume method was used with limited success. In one case the short-term total suspended targets were met but the 48 hour targets were surpassed. This crossing was also the largest flume project, encountered problems with unfiltered discharge water re-entering the watercourse and required eight days of instream activity.
In general, the degree of success at watercourses crossed using the flume method seems to be less than other crossing techniques. As one construction superintendent confessed, "he has done about a dozen, was only proud of one..."
Seven examples of watercourses crossed using the temporary diversion method were considered in the case history summary. All but two of the examples were on large rivers where alternative techniques to limit sedimentation of downstream areas were limited. Two of the examples required excavation of new channels in old high water or abandoned channels, one had an entirely new channel excavated between meanders in a silty floodplain, and the other four were diversions around islands and gravel bars using existing active channels.
Of those crossings which required excavation of a new channel, one was a last minute decision with no planning and no erosion protection of the new channel.
The other two were well planned and had sufficient geotextile and riprap onsite to prevent erosion of the new channel. Those crossings that used existing channels only had erosional concerns as a result of increased water velocity and depth. One example indicated that gravel displacement from a change in flow patterns was noted 900 m downstream of the diversion. At one crossing, flumes were installed to allow flow in the new channel to cross over the previously excavated trench.
Flumes were also installed at one crossing as a contingency in the new channel.
Of the three crossings where sedimentation and water quality observations were provided, results indicated that: water quality objectives were met; turbidity was not noticeable while constructing the dams; and only a minimal increase in silt load occurred due to heavy silt load already present in the river. On one crossing it was observed that sedimentation increased after diverting streamflow into an unlined new channel.
Generally the temporary diversions, if planned and implemented appropriately, were considered successful. The one crossing where difficulty was encountered was the result of a sudden change in methodology from the open cut trenching method to temporary diversion. Therefore, thorough planning of the procedure and appropriate protection measures were not in place. Difficulties that arose during construction of the crossings considered to be successful were problems associated with the efficient diversion of water; the erosion of the new channels;
and the correct placement of spoil so as to avoid susceptibility to erosion caused by increased volumes.
Five examples of two-stage coffer dams are summarized, although one reference is a generic reference to approximately 40 coffer dam crossings which were undertaken over a several year period and another is similar to five other crossings undertaken by the same construction superintendent.
All examples were constructed within large rivers between 25 m and 100 m in width, with substrates of coarse textured materials. Dams were constructed from various materials including clean pitrun, 1 m3 sandbags, washed gravel with plastic liner and conventional sandbags. At one crossing where 1 m3 sandbags were installed, an upstream deflection dam was also constructed to reduce the water velocity in the vicinity of the dam construction. Seepage and infiltration of water into the coffered area posed a problem in all cases. This was generally handled by installing numerous pumps. In one case, a sheet piling dam was installed inside the coffer and sealed with sand. Unfortunately, trench sloughing caused the sheet piling to fall into the trench and cables were installed to hold the sheet piling back. Riprap was installed at one watercourse on the upstream face of
Special environmental measures employed included downstream silt monitoring, and the installation of sorbent booms in the even of an accidental spill.
In general, coffer dams seem to work well as long as they are well planned and installed by an experienced crew. The engineering manager of the company which had completed 40 coffer dam crossings indicated that once the crew was experienced, construction was very successful. One superintendent also indicated great success once the system had been worked out but also indicated it was very costly and did increase the instream period. The expense was confirmed by one quote of $300,000 for a 100 m crossing. Many of those interviewed during the case history review indicated that they did not have any experience with this crossing method and noted strong reservations related to the mid stream tie-in due to safety and constructability. Two respondents indicated that they would only consider this technique in the event that instream repairs were required.
Horizontal Directional Drilling
Directional drilling can be an effective method for installing pipelines beneath watercourses with relatively low environmental impact to streambanks and water quality. Potential impacts associated with directionally drilled installations include land clearing affecting visual and wildlife values, possible loss of drill mud and the effect on water quality during construction as well as disposal of used drilling mud. The feasibility of using directional drilling techniques is strongly limited by site conditions, including soil characteristics, and available workspace and geometric constraints. The case history review indicated that drill mud seepage can occur for all soil types and is most likely when highly permeable zones are present with minimal cover between the drill path and the bed of the watercourse. There was a higher incidence of drill mud seepage for sites characterized by larger grain sized materials (gravels, cobbles and boulders) than for sites characterized by fine-grained and consolidated materials. The incidence of significant technical difficulty (i.e., loss of equipment, collapsed bore holes and
The significance of potential drill mud seepage into the watercourse is typically limited to point sources along the drill path. In some instances there is the opportunity to reduce or arrest seepage by decreasing the pressure of the drill mud. Depending on where these point sources occur, it may also be possible to implement mitigative measures such as containment berms and vacuum trucks to control water contamination. These measures can be effective for mud seepage occurring along the approach slopes and in some cases, shallow near-shore areas.
Significant leakage of drilling mud can also occur at the drill entry or exit point due to different pressure heads if there is a large change in elevation between the two points as well as during reaming or pull-back.
There are a number of site-specific engineering and geological constraints that may preclude the use of drilling as a viable crossing alternative. These include available workspace, pipeline specifications (length and diameter), site geometrics and soil conditions. The technology is particularly well suited for sites with finegrained soil characteristics (sands, silts and clay and consolidated soil types such as rock and sandstone. Unconsolidated materials with large gravels, cobbles and boulders are extremely difficult to drill and are one of the main limitations to directional drill applications. Potential problems with these materials include deflection of the drill bit, drill bit damage and equipment losses, removing boulders/cobbles from the bore, possible collapse of the bore hole and pipe damage during the pull-back operation. The potential for these problems generally increases with the size of the bore. Although directional drilled installations have been completed through mixtures of gravel, cobble and/or boulders, the installation failure rate and incidence of serious technical difficulties is high. This was particularly true for sites where large cobble and boulders were present. The number of successful installations through these conditions was relatively low.
These potential problems are further compounded for installations of large diameter pipes and increased crossing width. The small number of installations involving large diameter pipe identified in this review, coupled with the relatively high incidence of technical difficulty experienced further supports this conclusion.